The present invention relates to a method of manufacturing a semiconductor substrate in which a modified layer such as an impurity layer or dielectric layer is buried.
In semiconductor devices, in order to form a pn junction or an insulating layer, a semiconductor substrate is locally modified by an impurity diffusion or oxidation technique. Also, a metal, insulating material, semiconductor material or the like is deposited or grown on a semiconductor substrate by various thin film formation techniques. However, these techniques are generally used for diffusing an impurity from a surface of the semiconductor substrate into a portion inside thereof or depositing a thin film thereon. For this reason, it is difficult to bury a modified layer in the semiconductor substrate by these techniques.
For example, a method wherein oxygen is ion-implanted in a silicon substrate which is then annealed so as to form an oxide layer therein is known. However, in this method, when the oxide layer is buried in a deep portion of the silicon substrate, the silicon substrate is considerably damaged. In addition, since margins for ion implantation dose and depth control are small, it is very difficult to bury the oxide layer of a predetermined thickness precisely at a predetermined depth of the substrate.
Many devices, such as a bipolar IC, require a buried layer of a conductivity type and an impurity concentration different from those of the substrate. FIG. 1 shows a normal bipolar IC as an example of such a device. A manufacturing process of the bipolar IC is as follows. An n.sup.+ -type layer 12 as a prospective collector buried layer is formed in a p-type silicon substrate 11 by selective diffusion. An n.sup.- -type layer 13 is formed on the substrate 11 by an epitaxial growth method. A p-type base region 14 and an n.sup.+ -type source region 15 are sequentially formed in the n.sup.- -type layer 13 by a diffusion or ion-implantation method so as to obtain a bipolar transistor. Reference numeral 16 denotes a p.sup.+ -type diffusion layer for element isolation; and 17, an n.sup.+ -type diffusion region for a collector electrode.
According to such a conventional method of manufacturing the bipolar IC, the depth of the buried layer is determined by the thickness of the epitaxial layer 13. However, the epitaxial growth method is limited by its growth speed, since it takes a considerably long period of time to obtain a thick epitaxial layer. In addition, since the epitaxial growth method requires high skill and is difficult to control, defects and the like are easily formed in the epitaxial layer 13. In the epitaxial method requiring a long period of time, since an impurity in the buried layer 12 is redistributed, the depth of the buried layer 12, i.e., the thickness of the epitaxial layer 13 on the buried layer 12 is difficult to control.
In this manner, a structure of a semiconductor device is undesirably limited, and new techniques must be developed in order to realize a device design which satisfies requirements for device characteristics. Therefore, demand for a technique for burying a modified layer such as an impurity layer and an insulating layer in a semiconductor substrate with a simple process and high controllablity has arisen.